The present invention relates to electrohydraulic drives for vehicles, especially for locomotives. More particularly, the invention relates to improvements in drives of the type wherein a variable-speed electric motor transmits torque to a hydrodynamic power transmission which comprises a torque converter, a coupling and a hydrodynamic brake (the brake constitutes a second torque converter). The drive further comprises a control unit which automatically engages the torque converter or the coupling in such a way that the converter and the coupling are respectively operative in the lower and higher speed ranges.
The just described electrohydraulic drive exhibits many important advantages, compared with the normal electric drive actually used. Thus, and because the drive comprises a hydrodynamic brake, the dimensions of the electric motor need not be selected for operation as a generator during braking but solely for the purpose of insuring satisfactory traction. Moreover, it is possible to dispense with braking by means of the motor (i.e., the motor need not be operated as a generator) which, among other advantages, allows for deletion of braking resistors. Moreover, by engaging the torque converter during startup, the current rating (or intensity), of the motor during startup can be reduced with attendant reduction of thermal stresses upon the windings and commutator, provided that the motor embodies a commutator. This renders it possible to employ a small motor and to reduce the dimensions of the transformer, switchgear and other components.
In spite of the aforediscussed advantages of an electrohydraulic drive, such drives failed to gain acceptance in the industry. Instead of taking advantage of the aforediscussed features of drives embodying electric motors and hydrodynamic power transmissions, the manufacturers of locomotives, especially of those geared to high passenger train speeds, presently favor an entirely different approach. Replacement of the previously preferred 1-phase A.C. commutator motors or the more recent universal motors with commutator-free polyphase motors is known. This is attributable to recent developments in the electronic industry. Polyphase motors are relatively small and simple, especially owing to the omission of commutators which are subjected to extensive wear, and the tractive effort of such motors is superior to that of previously utilized motors. It has been found that a polyphase motor will furnish a high tractive effort in the lower speed range. In the higher speed range, and at a constant output, the tractive effort decreases with increasing speed. Therefore, such motors are less likely to be subjected to excessive thermal stresses, even during frequent startup. Consequently, a locomotive whose drive embodies a commutator-free polyphase motor can be used to pull high-speed passenger trains as well as extremely heavy low-speed freight trains. In other words, a locomotive drive which utilizes the just discussed motor or motors exhibits advantages which are similar to those of electrohydraulic drives; the only difference is that the hydrodynamic conversion of torque is replaced with electrical conversion of torque.
However, at the present time, the development of electronic equipment which is needed in locomotives employing drives with polyphase motors has not reached that stage which would render it possible to equip all electric locomotive drives with polyphase motors. The electronic equipment which must be used in combination with polyphase motors is extremely bulky and heavy so that the total weight of a locomotive whose drive embodies one or more polyphase motors is not substantially less than that of a locomotive whose drive employs conventional electric motors. Moreover, the cost of the electronic equipment for use with polyphase motors is prohibitive and, therefore, the decision to switch to locomotive drives which embody polyphase motors is not expected in the near future; such decision must be preceded by lengthy experimentation and testing.
The reluctance of experts in the field of locomotive drives to adopt electrohydraulic drives is attributable to the fact that an electrohydraulic drive must convert mechanical energy into fluid energy and the fluid energy back into mechanical energy. In other words, when compared with a device for direct transmission of mechanical energy, the efficiency of a hydrodynamic torque converter is relatively low. In addition, experts in the field of locomotives are reluctant to rely exclusively on a hydrodynamic brake, especially when the braking action must be supplied within the higher and maximum speed ranges. In other words, the experts are not convinced that a single dynamic brake suffices to provide the necessary safety factor during braking of a locomotive with a speed range of up to and possibly in excess of 300 km/h.